1. Field of the Invention
This invention relates to an offset printing ink composition.
2. Prior Art
Relief printing processes, lithographic printing processes including an offset printing process, and gravure printing processes have been widely used for preparing general printing matters. In the offset printing process, the inked image is once transferred from a print roller to a rubber blanket, and then printed on a sheet of paper. The print roller has a surface composed of hydrophilic portions (non-image portions) containing water and repelling oily ink and ink-holding portions (image portions) containing oil ink and repelling water. The most widely used offset printing inks are resin varnishes containing resins, such as alkylphenol resins, phenolic resins modified with rosin or maleic acid resins, which are dissolved in drying oils such as linseed oil. The phenolic resins modified with rosin are the most commonly used resins. Although a printing ink vehicle composed of a phenolic resin modified with rosin, a solvent and/or a drying oil is excellent in quality, it has disadvantages in that it is expensive and contains rosin which is a material of natural origin and unstable in supply. In place of the phenolic resin modified with rosin, it has been proposed to use so-called petroleum resins synthesized by polymerizing cracked oil fractions obtained as the byproducts of petroleum or petrochemical industries. Although these petroleum resins have advantages that the supply and cost thereof are stable, the qualities thereof are not satisfactory when used as the vehicle resins for the printing inks.
The principal properties, i.e. printability, required for the printing ink, particularly when used for an offset printing ink, are as follows:
(1) The ink must have an appropriate fluidity represented by the viscosity and the yield value. PA1 (2) A good balance should be retained at the interface between the hydrophilic portion and the ink-holding portion so as to form an image on a plane over which water and an ink contact with each other. PA1 (3) A pigment must be uniformly dispersed in the ink. PA1 (4) The printed face has a beautiful gloss and uniform printed matters are obtainable. PA1 (5) The time required for setting or drying is short and the ink does not suffer blocking. PA1 (6) The printed face has a resistance against friction. PA1 (1) It must have a high softening point but it should not have high molecular weight. PA1 (2) It must have a polar group to improve dispersibility of a pigment, namely it must have a polar group which has a good affinity with a pigment. PA1 (3) It must be easily soluble in a hydrocarbon solvent which has a high boiling point and contains only a small amount of an aromatic compound. PA1 (4) It must have a sufficient solubility in a drying oil, such as linseed oil.
In order to prepare an offset printing ink satisfying the aforementioned properties, the resin used therein must satisfy the following requirements.
Particularly, with the development of printing technology in recent years, it is required to increase the printing speed. In reply to this requirement, the offset rotary printing process is increasingly used. In the offset rotary printing process wherein the ink is heated to dryness, it is eagerly desired to use a high boiling point hydrocarbon solvent, such as a paraffin solvent, which contains an aromatic compound in an amount as small as possible, preferably no aromatic compound, to improve the environment at the working sites and to exclude the pollution problem caused by the exhaust gases. However, there has not yet been developed an offset printing ink having a satisfactory printability, as described above. Accordingly, there is an increasing demand for an offset printing ink having a satisfactory printability.
It has been already known from U.S. Pat. No. 3,084,147 that a resin soluble in a hydrocarbon solvent, such as benzene, toluene, xylene, cyclohexane or isooctane, can be prepared by thermally polymerizing cyclopentadiene at a high temperature of from 250.degree. to 350.degree. C. in the presence of an inert hydrocarbon solvent, such as benzene, toluene, xylenene or isooctane. The dicyclopentadiene resin prepared by this known method is soluble in a hydrocarbon solvent including benzene, toluene, xylene, solvent naphtha and Solvent No. 5 (Trade name produced by Nippon Oil, Co., Ltd.). However, since this dicyclopentadiene resin has no polar group, it cannot be applied for a variety of uses due to its poor miscibility and poor adhesive property to other materials. An ink prepared by adding various solvents and pigments to the aforementioned resin cannot be used as an offset printing ink, since the dispersibility of pigments in the ink is unsatisfactory, leading to uneven printed face having bad gloss.
Another proposal has been made by Japanese Patent Provisional Publication No. 24405/1972, wherein an acrylic ester or maleic acid anhydride is added to said dicyclopentadiene resin followed by hydrolysis to prepare a carboxylic acid-containing resin, which is then reacted with a polyhydric alcohol and a higher unsaturated carboxylic acid to produce a resin for printing inks. However, the resin prepared in accordance with this proposal and having a high softening point to give an offset printing ink to be set and dried within a time substantially equal to that required for setting and drying the conventional offset printing inks, is gotten poor in solubility in a hydrocarbon solvent originated from petroleum which is used as the solvent for the offset printing ink. As the results, the fluidity of the ink prepared therefrom is gotten poor and the gloss of the ink becomes extremely worse. A further disadvantage of this resin is that it forms gels in the step of the preparation of a varnish to give a cloudy varnish having a bad hue. On the contrary, the softening point of this resin must be lowered to improve the solubility in the solvent, resulting in prolongation of time required for setting and drying. For the aforementioned reasons, the resin prepared in accordance with the prior proposal cannot be used in an offset printing ink for practical use.
Alternatively, U.S. Pat. No. 2,608,550 discloses a resin prepared by thermally copolymerizing dicyclopentadiene and maleic acid anhydride. However, the resin taught by this prior Patent and having a relatively low content of maleic acid has, generally, an inconveniently high softening point and is poor in dispersibility of pigments and also poor in miscibility with other filler materials. If the content of maleic acid anhydride is increased to overcome the aforementioned disadvantages, there appears a tendency of coloring and gellation and the resultant resin becomes poor in weatherproof property.
U.S. Pat. No. 2,608,550 also discloses an alkyd resin produced from a so-called alkyd composition including a polybasic acid component consisting of a resin prepared by thermally polymerizing dicyclopentadiene with maleic acid anhydride. However, the alkyd resins taught by this prior patent have, in general, high molecular weights, poor solubilities in high boiling point hydrocarbon solvents for the printing inks and in drying oils. As the results, a printing ink prepared from any of these alkyd resins is poor in fluidity, inferior in gloss of the printed matters, and tends to scatter in the form of mists from the print roller to stain papers, resulting in serious misting problem. For these reasons, the alkyd resins taught by the prior Patent referred to hereinabove cannot be applied for practical use.
Although a drying oil, such as linseed oil, has been used in the conventional offset printing ink varnish, the used quantity thereof tends to be decreased with the increase in printing speed to obviate the problems of stain of the ground region caused by the use of a drying oil and to obviate occurrence of misting and retardation of setting.
The conventional offset printing ink varnish coprises a high boiling point hydrocarbon solvent, an oily component such as a drying oil, and a binder resin. As the binder resin for this purpose, alkylphenol resins, phenolic resins modified with rosin and maleic acid resins have been conventionally used. However, a varnish composition containing a decreased amount of a drying oil does not give glossy and beautiful printed matters even when any of the known binder resins is added.
In addition, any of the known resins is far from satisfactory in solubility in a paraffin solvent, so that they are almost useless as the binder resins in offset printing inks dissolved in paraffin solvents.